Method for producing flexo printing forms by means of laser-direct engraving

ABSTRACT

Flexographic printing plates are produced by means of direct laser engraving by a process in which the starting material used is a flexographic printing element which has a relief-forming layer comprising a combination of a styrene/butadiene block copolymer and 20-40% by weight of a plasticizer. Flexographic printing plates obtainable by this process are used for flexographic printing with water-based or alcohol-based printing inks.

[0001] The present invention relates to a process for the production offlexographic printing plates by means of direct laser engraving, inwhich the starting material used is a flexographic printing elementwhich has a relief layer comprising a combination of a styrene/butadieneblock copolymer and 20-40% by weight of a plasticizer. The presentinvention furthermore relates to flexographic printing plates obtainableby this process and the use of flexographic printing plates forflexographic printing with water-based or alcohol-based printing inks.

[0002] Lasers are now used both in the area of offset printing platesand in the area of relief printing plates for various steps of theproduction process.

[0003] For example, it is known that the photosensitive layers of offsetprinting plates can be inscribed imagewise by means of suitable laserexposure units. The photosensitive layer is chemically modified, forexample crosslinked, by the laser. The finished offset printing plate isobtained from the image-bearing crude product by means of a suitabledevelopment process (cf. for example Imaging Technology, Section3.4.1.2., Ullmann's Encyclopedia of Industrial Chemistry, 6^(th) Edt.,2000 Electronic release). The thickness of said photosensitive layers ofoffset printing plates is usually from 0.3 to 5 μm.

[0004] It is furthermore known that images can be produced fromflexographic printing plates with the use of IR-ablative masks, asdisclosed, for example, in EP-A 654 150, instead of photographicallyproduced masks. Here, a thin IR-sensitive, opaque layer is applied tothe photopolymerizable layer. The thickness of such IR-ablative layersis usually just a few μm. The IR-ablative layer is inscribed imagewiseusing an IR laser, i.e. the parts in which the laser beam is incident onit are removed. The actual printing relief is produced in theconventional manner: exposure is effected to actinic light through themask produced, and the relief layer is thus selectively crosslinked.Development is then effected with a washout agent in a conventionalmanner, both photosensitive material from the unexposed parts of therelief-forming layer and the residues of the IR-ablative layer beingremoved. Since the IR-ablative mask layer is of no importance for theactual printing process, the materials therefor can be soughtexclusively with regard to the optimum use as a mask.

[0005] In direct laser engraving for the production of flexographicprinting plates, on the other hand, a printing relief is engraveddirectly into the relief layer of a flexographic printing element bymeans of a laser. A subsequent development step, as in the conventionalprocess or in the mask process, is no longer required. Typical relieflayer thicknesses of flexographic printing plates are from 0.5 to 7 mmand may also be 0.2 mm in the case of special thin-film plates. Thenonprinting wells in the relief are at least 0.03 mm in the screen areaand substantially more in the case of other negative elements and mayassume values up to 3 mm in the case of thick plates. Thus, largeamounts of material have to be removed by means of the laser.

[0006] EP-A 640 043 and EP-A 640 044 disclose one-layer or multilayerelastomeric laser-engravable flexographic printing elements for theproduction of flexographic printing plates by means of laser engraving.The elements consist of reinforced elastomeric layers. Elastomericbinders are used for the production of the layer. The mechanicalstrength of the layer is increased by the reinforcement, in order topermit flexographic printing. The reinforcement is achieved either byintroduction of suitable fillers, photochemical or thermochemicalcrosslinking or combinations thereof.

[0007] U.S. Pat. No. 5,259,311 discloses a process in which a commercialflexographic printing element is photochemically crosslinked by uniformexposure to UV/A in a first step, the release layer is then removedusing a flexographic washout agent and a printing relief is engraved bymeans of a laser in a second step. A cleaning step is then carried outby means of a flexographic washout agent, followed by final drying ofthe plate.

[0008] Although the engraving of rubber impression cylinders by means oflasers has in principle been known since the 60s of the last century andthe patents cited have also been filed 10 years ago, laser engraving hasacquired broader commercial interest only in recent years with theadvent of improved laser systems. The improvements in the laser systemsinclude better focusability of the laser beam, higher power andcomputer-controlled beam modulation.

[0009] With the introduction of new, more efficient laser systems,however, the question of particularly suitable materials forlaser-engravable flexographic printing plates is becoming increasinglyimportant. Problems which played no role at all in the past because thelaser systems did not at all allow the engraving of very fine structuresare now important and lead to new requirements with respect to thematerial.

[0010] The relief layers of flexographic printing plates are of coursesoft and have relatively low melting or softening points. In laserengraving, they therefore have a strong tendency to form melt edgesaround the engraved elements. At the edge of the engraved elements, thelayer melts under the influence of the laser beam but is not, or notcompletely, decomposed. Such melt edges cannot be removed or at leastcannot be completely removed even by subsequent washing and lead to ablurred print. Undesired melting of the layer furthermore results inreduced resolution of the print motif in comparison with the digitaldata record.

[0011] EP-A 1 136 254 proposes the use of relief layers comprisingpolyoxyalkylene/polyethylene glycol graft copolymers as binders forsolving this problem. However, since these copolymers are water-soluble,such relief printing plates have the disadvantage that they can be usedonly to a limited extent. The relief layer swells to an excessive extentin water-based flexographic printing inks, so that undesired effects,for example an intolerable increase in tonal value, occur duringprinting. Such printing plates can therefore be used substantially onlyfor printing with UV inks. There is an urgent need to providelaser-engravable relief printing elements which are also suitable forprinting with water-based inks and nevertheless can be engraved withlasers without undesired melting of the layer.

[0012] Furthermore, the degradation products which form in the course ofthe laser engraving frequently give rise to problems. In addition togaseous fractions, aerosols are also produced. These are as a ruleextremely tacky and may be wholly or partly deposited again on thesurface of the printing relief and, in unfavorable cases, can even reactagain with the surface. This leads to unclean surfaces and hence also topoor printing behavior.

[0013] For solving this problem, U.S. Pat. No. 5,259,311 proposessubsequently cleaning the surface of the relief printing plate after thelaser engraving with the aid of an organic solvent. However, the tackydecomposition products have substantially the same solubility behavioras the relief layer. For relief layers comprising hydrophobic polymers,an organic solvent therefore also has to be used for removing thedecomposition products. The crosslinked relief-forming layer is nolonger soluble therein but may well still be swellable. After such asubsequent washing step, the layer therefore has to be dried again in afurther process step. The time and handling advantage achieved by laserengraving in the process is eliminated again since the drying processtakes the most time in the course of processing. Decomposition productswhich have reacted again with the surface can no longer be removed atall and are consequently also detectable in the print. It will beextremely desirable to be able to have a flexographic printing elementin which possible deposits can be removed simply with water or aqueouscleaning agents without the plate swelling thereby.

[0014] Very rapid engraving is furthermore required for the economicalproduction of flexographic printing plates by means of laser engraving.The speed of the engraving depends on the one hand on the laser systemchosen. On the other hand, the sensitivity of the relief-forming layerto the laser radiation chosen in each case should be very high. Withregard to the sensitivity, however, it should be taken into account thatthe relief layer of the flexographic printing plate imparts both theelastomeric properties and the typical printing properties. Measures forimproving the sensitivity therefore must not impair said properties.

[0015] It is an object of the present invention to provide a process forthe production of flexographic printing plates by means of direct laserengraving, in which the occurrence of melt edges is substantiallyreduced, a very small amount of aerosols forms and possible deposits ofdecomposition products can be removed by simple treatment of the platewith water or aqueous cleaning agents and which process permits veryrapid engraving with high resolution and in which the flexographicprinting plates obtained are moreover suitable for printing withwater-based flexographic printing inks.

[0016] We have found that this object is achieved by a process for theproduction of flexographic printing plates by means of laser engraving,in which the starting material used is a crosslinkable, laser-engravableflexographic printing element which at least comprises, arranged one ontop of the other,

[0017] a dimensionally stable substrate,

[0018] at least one crosslinkable, laser-engravable relief layer havinga thickness of at least 0.2 mm, at least comprising an elastomericbinder, a plasticizer and crosslinkable components which processcomprises at least the following steps:

[0019] (a) uniform crosslinking of the relief-forming layer and

[0020] (b) engraving of a printing relief into the crosslinkedrelief-forming layer with the aid of a laser, the height of the reliefelements engraved with the laser being at least 0.03 mm,

[0021] the binder being a styrene/butadiene block copolymer having anaverage molecular weight M_(w) of from 100 000 to 250 000 g/mol, a ShoreA hardness of from 55 to 85 and a styrene content of 20-40% by weight,based on the binder, and the amount of the plasticizer being from morethan 20 to 40% by weight, based on the sum of all components of thelayer.

[0022] Flexographic printing plates which are obtainable by the processdescribed and the use of these flexographic printing plates forflexographic printing with water-based and/or alcohol-based printinginks have furthermore been found.

[0023] Surprisingly, it has been found that flexographic printingelements which have excellent sensitivity to lasers are obtained by thenovel combination of styrene/butadiene block copolymers with from 20 to40% by weight of plasticizers. The relief-forming layer scarcely meltsunder the influence of the laser radiation, and scarcely any melt edgesform around the negative elements. The flexographic printing platesobtained also permit printing with water-based and/or alcohol-based inkswithout the relief layer swelling excessively with these inks.

[0024] Regarding the present invention, the following may be statedspecifically:

[0025] Examples of suitable dimensionally stable substrates for theflexographic printing elements used as starting materials for theprocess are plates, sheets and conical and cylindrical sleeves ofmetals, such as steel, aluminum, copper or nickel, or of plastics, suchas polyethylene terephthalate (PET), polyethylene naphthalate (PEN),polybutylene terephthalate, polyamide, polycarbonate, if required alsowoven fabrics and nonwovens, such as glass fiber fabrics, and compositematerials, for example of glass fibers and plastics. Particularlysuitable dimensionally stable substrates are dimensionally stablesubstrate sheets, for example polyester sheets, in particular PET or PENsheets, or flexible metallic substrates, such as thin metal sheets ormetal foils of steel, preferably of stainless steel, magnetizable springsteel, aluminum, zinc, magnesium, nickel, chromium or copper.

[0026] The flexographic printing element furthermore comprises at leastone laser-engravable, crosslinkable relief-forming layer. Thecrosslinkable relief layer may be applied directly on the substrate.However, other layers, for example adhesion-promoting layers and/orresilient lower layers, may also be present between the substrate andthe relief layer.

[0027] The crosslinkable relief-forming layer comprises at least oneelastomeric binder, crosslinkable components and from 20 to 40% byweight of a plasticizer. As a rule, the crosslinkable relief layer as awhole already has elastomeric properties; for the present invention,however, it is sufficient if the crosslinked relief layer first has theelastomeric properties typical of a flexographic printing plate.

[0028] According to the invention, the elastomeric binder is astyrene/butadiene block copolymer. It may be a two-block copolymer,three-block copolymer or multiblock copolymer in which in each case aplurality of styrene and butadiene blocks follow one anotheralternately. It may be a linear, branched or star block copolymer.Preferably, the block copolymers used according to the invention arestyrene/butadiene/styrene three-block copolymers. Such SBS blockcopolymers are commercially available, for example under the nameKraton®, it being necessary to take into account the fact thatcommercial three-block copolymers usually have a certain content oftwo-block copolymers. Of course, mixtures of different SBS blockcopolymers can also be used.

[0029] The elastomeric styrene/butadiene block copolymers used accordingto the invention in the starting material have an average molecularweight M_(w) (weight average) of from 100 000 to 250 000 g/mol. M_(w) ispreferably from 150 000 to 250 000, very particularly preferably from150 000 to 200 000, g/mol.

[0030] The styrene content of the styrene/butadiene block copolymersused is from 20 to 40, preferably from 25 to 35, % by weight, based onthe binder.

[0031] The Shore A hardness of the binder is determined by the method ofISO 868. According to the invention, the elastomeric styrene/butadieneblock copolymer used has a hardness of from 55 to 85 Shore A. Thehardness of the binder is particularly preferably from 60 to 80, veryparticularly preferably from 65 to 75, Shore A.

[0032] In addition to the at least one styrene/butadiene blockcopolymer, the relief layer can optionally also have one or moresecondary binders. Such secondary binders can be used by a personskilled in the art for fine control of the properties of the relieflayer. The choice of secondary binders is in principle not limited,provided that the properties of the relief layer are not adverselyaffected thereby. Secondary binders are preferably styrene/butadieneblock copolymers which do not only meet the abovementioned requirementswith regard to molecular weight, hardness and styrene content. However,there may of course also be polymers of a chemically different type. Theamount of secondary binder should as a rule not exceed 20, preferably10, % by weight, based on the total amount of all binders used. If thesecondary binder is a styrene/butadiene block copolymer, up to about 30%by weight, in special cases also up to about 40% by weight, based on thetotal amount of all binders used, may be employed.

[0033] The total amount of binders, i.e. styrene/butadiene blockcopolymers and any secondary binders present together, is usually from40 to 80, preferably from 40 to 70, particularly preferably from 45 to65, % by weight, based on the sum of all components of therelief-forming layer.

[0034] For the novel process, the binder is used as a mixture with atleast one plasticizer. The amount of plasticizer is from 20 to 40,preferably from 25 to 40, particularly preferably from 30 to 40, % byweight, based on all components of the relief-forming layer.

[0035] The person skilled in the art chooses suitable plasticizersaccording to the desired properties of the relief layer. Examples ofsuitable plasticizers include modified and unmodified natural oils andnatural resins, such as high-boiling paraffinic, naphthenic or aromaticmineral oils, synthetic oligomers or resins, such as oligostyrene,oligomeric styrene/butadiene copolymers, oligomerica-methylstyrene/p-methylstyrene copolymers, liquid oligobutadienes, inparticular those having a molecular weight of from 500 to 5 000 g/mol,or liquid oligomeric acrylonitrile/butadiene copolymers or oligomericethylene/propylene/diene rubbers.

[0036] Inert plasticizers are particularly suitable for the novelprocess. Inert in the context of this invention means that theplasticizers have no or only substantially no polymerizable groups whichcan react in the course of free radical crosslinking of therelief-forming layer so that the plasticizers are also incorporated intothe polymeric network of the relief layer. Inert plasticizers have inparticular substantially no ethylenically unsaturated double bonds.

[0037] Examples of inert plasticizers include high-boiling paraffinic,naphthenic and aromatic mineral oils. Paraffinic and/or naphthenicmineral oils are substantially preferred. Such mineral oils can also bereferred to as white oils, a person skilled in the art making adistinction between technical-grade white oils, which may still have alow content of aromatics, and medicinal white oils, which aresubstantially free of aromatics.

[0038] It is of course also possible to use mixtures of differentplasticizers, provided that the properties of the relief layer are notadversely affected thereby. Preferred mixtures are those which compriseat least one inert plasticizer. An example is a mixture of liquidoligobutadienes and white oil.

[0039] The type and amount of the components for the crosslinking of thelayer depend on the desired crosslinking technique and are chosenaccordingly by a person skilled in the art. The uniform crosslinking ofthe crosslinkable relief layer is preferably carried outphotochemically, thermochemically or by means of electron beams.

[0040] In the case of the photochemical crosslinking, the relief layercomprises at least one photoinitiator or a photoinitiator system andsuitable monomers or oligomers.

[0041] Benzoin and benzoin derivatives, such as α-methylbenzoin andbenzoin ethers, benzil derivatives, such as benzil ketals,acylarylphosphine oxides, acylarylphosphinic esters and polynuclearquinones are suitable in a known manner as initiators for thephotopolymerization, there being no intention to restrict the list tothese.

[0042] The monomers have at least one polymerizable, olefinicallyunsaturated group. Esters or amides of acrylic acid or methacrylic acidwith mono- or polyfunctional alcohols, amines, aminoalcohols orhydroxyethers and hydroxyesters, styrene or substituted styrenes, estersof fumaric or maleic acid or allyl compounds have proven particularlyadvantageous. Examples of suitable monomers include butyl acrylate,2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanediol diacrylate,1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, 1,9-nonanedioldiacrylate, trimethylolpropane triacrylate, dioctyl fumarate andN-dodecylmaleimide. Suitable oligomers having olefinic groups may alsobe used. It is of course also possible to use mixtures of differentmonomers or oligomers, provided that no undesired effects occur. Thetotal amount of the monomers is established by a person skilled in theart according to the desired properties of the relief layer. As a rule,however, 20% by weight, based on the amount of all components of thelaser-engravable relief-forming layer, should not be exceeded.

[0043] Thermal crosslinking is preferably carried out analogously to thephotochemical crosslinking, by using a thermal polymerization initiatorinstead of a photoinitiator. Commercial thermal initiators for freeradical polymerization, for example peroxides, hydroperoxides or azocompounds, are in principle suitable. The thermal crosslinking may alsobe carried out by adding a heat-curable resin, for example an epoxyresin, as a crosslinking component to the layer.

[0044] Crosslinking by means of electron beams is preferably carried outanalogously to the photochemical crosslinking, by using photochemicallycrosslinkable relief layers described above and replacing the UVradiation with electron beams. The addition of initiators is notabsolutely essential.

[0045] The crosslinkable relief layer can optionally furthermorecomprise an absorber for laser radiation. Mixtures of differentabsorbers for laser irradiation may also be used. Suitable absorbers forlaser radiation have a high absorption in the region of the laserwavelength. Particularly suitable absorbers are those which have a highabsorption in the near infrared and in the longer-wave VIS range of theelectromagnetic spectrum. Such absorbers are particularly suitable forthe absorption of the radiation of Nd-YAG lasers (1 064 nm) and of IRdiode lasers, which typically have wavelengths of from 700 to 900 nm andfrom 1 200 to 1 600 nm.

[0046] Examples of suitable absorbers for laser radiation are dyes whichabsorb strongly in the infrared spectral range, for examplephthalocyanines, naphthalocyanines, cyanines, quinones, metal complexdyes, such as dithiolenes, or photochromic dyes. Further suitableabsorbers are inorganic pigments, in particular intensely coloredinorganic pigments, for example chromium oxides, iron oxides, carbonblack or metallic particles.

[0047] Particularly suitable absorbers for laser radiation are finelydivided carbon black grades having a primary particle size of from 10 to50 nm.

[0048] The amount of the optionally added absorber is chosen by a personskilled in the art according to the respective desired properties of thelaser-engravable flexographic printing element. In this context, aperson skilled in the art will take into account the fact that the addedabsorber influences not only the engraving of the elastomeric layer bylaser but also the properties of the relief printing plate obtained asthe end product of the process, for example its hardness, resilience,thermal conductivity or ink transfer behavior. As a rule, it istherefore advisable to use not more than 20% by weight at most,preferably not more than 10% by weight, based on the sum of allcomponents of the layer, of absorber for laser radiation.

[0049] As a rule, it is not advisable to add to relief layers which areto be photochemically crosslinked absorbers for laser radiation whichalso absorb in the UV range, since the photopolymerization is at leastgreatly impaired thereby and may be rendered completely impossible. Itis advisable as a rule to subject such layers containing laser absorbersto thermal crosslinking or crosslinking by means of electron beams.

[0050] The relief-forming layers furthermore comprise additives andassistants, for example dyes, dispersants or antistatic agents. However,the amount of such additives should as a rule not exceed 5% by weight,based on the amount of all components of the crosslinkable,laser-engravable layer of the recording element.

[0051] The crosslinkable relief-forming layer may also be composed of aplurality of part-layers. These crosslinkable part-layers may be of thesame, roughly the same or different material composition.

[0052] The thickness of the laser-engravable, elastomeric layer is atleast 0.2, preferably from 0.3 to 7, particularly preferably from 0.5 to5, very particularly preferably from 0.7 to 4, mm. The thickness issuitably chosen by a person skilled in the art according to the desireduse of the flexographic printing plate.

[0053] In a preferred embodiment, the starting material comprises anadditional laser-engravable polymer layer which is soluble or at leastswellable in aqueous media and is arranged on the laser-engravablerelief-forming layer, and which comprises at least one polymer soluble,swellable or dispersible in aqueous solvents. Such a layer serves forfacilitating a subsequent cleaning step optionally to be carried out.Solid decomposition products formed in the course of the laser engravingmay be deposited on this auxiliary layer and can be more easily removed.

[0054] Examples of the polymer soluble or at least swellable in aqueoussolvents include polyvinyl alcohol, polyvinyl alcohol/polyethyleneglycol graft copolymers, polyvinylpyrrolidone and its derivatives andcellulose derivatives, in particular cellulose esters and celluloseethers, such as methylcellulose, ethylcellulose, benzylcellulose,hydroxyalkylcelluloses or nitrocelluloses. Mixtures of a plurality ofpolymers can of course also be used.

[0055] The additional laser-engravable polymer layer may also containadditives and assistants, for example plasticizers or laser absorbers.If it is intended to crosslink the laser-engravable relief layerphotochemically, the additional polymer layer should as far as possiblebe transparent in the UV range. In the case of other crosslinkingmethods, this is not absolutely essential.

[0056] The thickness of the additional polymer layer should be verysmall. It depends substantially on the depth of focus of the laser usedfor engraving in the process. It is limited so that there is nosubstantial broadening of the focus on the surface of the relief layer.

[0057] The thickness of such an additional polymer layer should as arule not exceed 100 μm. As a rule, satisfactory results are no longerachieved in the case of greater thicknesses. The thickness shouldpreferably not exceed 50 μm. The thickness is particularly preferably1-40 μm, very particularly preferably 2-25 μm.

[0058] The laser-engravable flexographic printing element can optionallyalso comprise further layers.

[0059] Examples of such layers include elastomeric lower layerscomprising a different formulation, which is present between thesubstrate and the laser-engravable layer or layers and which need notnecessarily be laser-engravable. The mechanical properties of the reliefprinting plates can be modified by means of such lower layers withoutthe properties of the actual printing relief layer being influenced.

[0060] Resilient substructures which are present under the dimensionallystable substrate of the laser-engravable flexographic printing element,i.e. on that side of the substrate which faces away from thelaser-engravable relief layer, serve the same purpose.

[0061] Further examples include adhesion-promoting layers which bond thesubstrate to layers located above or bond different layers to oneanother.

[0062] Furthermore, the laser-engravable flexographic printing elementcan be protected from mechanical damage by a protective sheet—also knownas covering sheet—which consists, for example, of PET and is present onthe respective uppermost layer and which has to be removed beforeengraving by means of lasers. To facilitate peeling off, the protectivesheet may have been surface-treated in a suitable manner, for example bysiliconizing, provided that the top relief layer is not adverselyaffected in its printing properties by the surface treatment.

[0063] The flexographic printing element used as a starting material forthe process can be produced, for example, by dissolving or dispersingall components in a suitable solvent and casting on a substrate. In thecase of multilayer elements, a plurality of layers can be cast one ontop of the other in a manner known in principle. After the casting, thecover sheet can, if desired, be applied for protecting the startingmaterial from damage. Conversely, it is also possible to cast onto thecover sheet and finally to laminate with the substrate. The castingmethod is particularly advisable if thermal crosslinking is intended.

[0064] If photochemical or electron beam crosslinking is intended, theproduction of the relief layer is preferably carried out in a mannerknown in principle by melt extrusion between a substrate sheet and acover sheet or a cover element and calendering of the compositeobtained, as disclosed, for example, in EP-A 084 851. In this way, it isalso possible to produce thick layers in a single operation. Multilayerelements can be produced, for example, by means of coextrusion.Flexographic printing elements having metallic substrates can preferablybe obtained by casting or extruding onto a temporary substrate and thenlaminating the layer with the metallic substrate.

[0065] It has usually proven useful first to process thestyrene/butadiene block copolymer with a part of the plasticizer in asuitable mixing unit to give a homogeneous material. The materialobtained is then further processed in a second step in an extrudertogether with the other components of the layer and the remainder of theplasticizer. A larger amount of plasticizer can advantageously thus alsobe incorporated over a short extruder length and particularlyhomogeneous incorporation of the plasticizer can be achieved. Moreover,the residence times of the polymeric material in the hot zone of theextruder can be reduced.

[0066] The application of the additional polymer layer can be effected,for example, by dissolving the components in a suitable solvent andcasting onto the relief-forming layer. Preferably, however, the coversheet is coated with the additional polymer layer and laminated with therelief-forming layer or used as a sheet for the extrusion process.

[0067] In the novel process, the starting material is first uniformlycrosslinked in the first process step (a).

[0068] The uniform crosslinking of the crosslinkable relief layer can becarried out photochemically, in particular by exposure to UV-A radiationhaving a wavelength of from 320 to 400 nm or UV-A/VIS radiation having awavelength of from about 320 to about 700 nm. Uniform thermochemicalcrosslinking is effected by very uniform heating of the relief layer atconstant temperature. Furthermore, crosslinking can be effected byuniform exposure to electron beams. The radiation dose required forcrosslinking can particularly advantageously be divided into a pluralityof part-doses.

[0069] The photochemical crosslinking is particularly suitable forrelief layers which contain no strongly colored absorbers for laserradiation and are transparent or at least substantially transparent inthe UV/VIS range. However, transparent relief layers can of course alsobe crosslinked thermochemically or by means of electron beams. Relieflayers containing colored laser absorbers can advantageously becrosslinked thermochemically or by means of electron beams.

[0070] Of course, the flexographic printing element used as a startingmaterial for the process is usually produced by a printing platemanufacturer whereas the laser engraving is carried out by processengravers or printing works. The uniform crosslinking (a) can on the onehand be carried out by the process engravers themselves. For example,the photochemical crosslinking can be carried out in commercial exposureunits for flexographic printing plates. On the other hand, thecrosslinking can of course also be effected by the manufacturer offlexographic printing elements or on his premises.

[0071] In process step (b), a printing relief is engraved into thecrosslinked relief layer by means of a laser. If a protective sheet ispresent, this is removed prior to engraving.

[0072] The term laser-engravable is to be understood as meaning that therelief layer has the property of absorbing laser radiation, inparticular the radiation of an IR laser, so that it is removed or atleast detached in those parts where it is exposed to a laser beam ofsufficient intensity. The layer is preferably vaporized or thermally oroxidatively decomposed without melting beforehand, so that itsdecomposition products are removed from the layer in the form of hotgases, vapors, fumes or small particles.

[0073] IR lasers are particularly suitable for engraving. For example, aCO₂ laser having a wavelength of 10.6 μm may be used. Furthermore,Nd-YAG lasers (1 064 nm), IR diode lasers or solid-state lasers may beused. It is also possible to use lasers having shorter wavelengths,provided that the laser has a sufficient intensity.

[0074] For example, a frequency-doubled (532 nm) or frequency-tripled(355 nm) Nd-YAG laser or an excimer laser (e.g. 248 nm) may also beused.

[0075] The addition of absorbers for laser radiation dependssubstantially on the type of laser which is to be used for theengraving. The styrene/butadiene block copolymers used for the relieflayer absorb the radiation of CO₂ lasers to a sufficient extent, so thatadditional IR absorbers in the relief layer are as a rule not requiredwhen this type of laser is used. The same applies to UV lasers, forexample excimer lasers. In the case of Nd-YAG lasers and IR diodelasers, the addition of a laser absorber is generally necessary.

[0076] The image information to be engraved can be transferred directlyfrom the layout computer system to the laser apparatus. The lasers canbe operated either continuously or in pulsed mode.

[0077] Relief elements in which the sidewalls of the elements initiallydrop perpendicularly and broaden only in the lower region areadvantageously engraved. A good shoulder shape of the relief dotstogether with little increase in tonal value is thus achieved. However,sidewalls of other designs can also be engraved.

[0078] The height of the elements to be engraved depends on the totalthickness of the relief and on the type of elements to be engraved andis determined by a person skilled in the art according to the desiredproperties of the printing plate. The height of the relief elements tobe engraved is at least 0.03 mm, preferably at least 0.05 mm, theminimum depth between individual dots being mentioned here. Printingplates having relief heights which are too small are as a ruleunsuitable for printing by means of a flexographic printing technique,because the negative elements become full to overflowing with printingink. Individual negative dots should usually have greater depths; forthose of 0.2 mm diameter, a depth of at least from 0.07 to 0.08 mm isusually advisable. In the case of surfaces which have been removed byengraving, a depth of more than 0.15 mm, preferably more than 0.4 mm, isadvisable. The latter is of course possible only in the case of anappropriately thick relief.

[0079] Advantageously, the flexographic printing plate obtained iscleaned in a further process step (c) after the laser engraving. In somecases, this can be effected by simply blowing off with compressed air orbrushing off.

[0080] In a preferred embodiment, a liquid cleaning agent is used forthe subsequent cleaning, in order also to be able to remove polymerfragments completely. This is particularly advisable, for example, whenfood packaging which has to meet particularly stringent requirementswith respect to volatile components is to be printed using theflexographic printing plate.

[0081] The subsequent cleaning can be very particularly advantageouslyeffected by means of water or an aqueous cleaning agent. Aqueouscleaning agents substantially comprise water and optionally smallamounts of alcohols and may contain assistants, for example surfactants,emulsifiers, dispersants or bases, for promoting the cleaning process.It is also possible to use mixtures which are usually used fordeveloping conventional, water-developable flexographic printing plates.Since the relief layer comprising styrene/butadiene block copolymers isnot swellable in water, time-consuming drying of the printing plate isavoided by the use of water or aqueous cleaning agents.

[0082] The subsequent cleaning can be effected, for example, by simpleimmersion or spraying of the relief printing plate or can additionallybe promoted by mechanical means, for example by brushing or treatmentwith a plush pad. It is also possible to use conventional flexographicplate washers.

[0083] In the subsequent washing step, any deposits and the residues ofthe additional polymer layer are removed. This layer advantageouslyprevents polymer droplets formed in the course of the laser engravingfrom becoming firmly bonded again to the surface of the relief layer, orat least makes it more difficult for this to occur. Deposits cantherefore be particularly readily removed. It is as a rule advisable tocarry out the subsequent washing step immediately after the laserengraving step.

[0084] Although not the preferred variant, it is also possible inprinciple to use mixtures of organic solvents for the subsequentcleaning, in particular those mixtures which usually serve as washoutagents for conventionally produced flexographic printing plates.Examples include washout agents based on high-boiling, dearomatizedmineral oil fractions, as disclosed, for example, in EP-A 332 070, orwater-in-oil emulsions, as disclosed in EP-A 463 016. This variant canbe used in particular when no additional polymer layer is present. If anadditional polymer layer is present but cannot be removed with theorganic solvent used, cleaning must additionally be effected with wateror an aqueous cleaning agent.

[0085] The flexographic printing plates obtained are particularlysuitable for printing with water-based inks and alcohol-based inks.However, they are of course also suitable for printing with UV inks orflexographic printing inks which contain small amounts of esters.

[0086] The examples which follow illustrate the invention:

EXAMPLE 1

[0087] A photochemically crosslinkable laser-engravable relief-forminglayer was produced using the following starting materials AmountComponent Description [% by wt.] Styrene/ SBS block copolymer, M_(w) 125000 g/mol, 55% butadiene 29.5% styrene content, 70° Shore A (Kratonblock D-1102) copolymer Plasticizer Polybutadiene oil 32% ComponentsMonomer: Hexanediol diacrylate 10% for Photoinitiator 2% crosslinkingAdditives Dye, thermal stabilizer 1%

[0088] The components were processed using an extruder (ZSK 53) at 140°C., introduced by means of a slot die between a dimensionally stable PETsubstrate sheet and a PET protective sheet and then calendered by meansof a two-roll calender. The thickness of the resulting crosslinkable,laser-engravable layer was 1.14 mm.

EXAMPLE 2

[0089] A photochemically crosslinkable laser-engravable relief-forminglayer was produced using the following starting materials AmountComponent Description [% by wt.] Styrene/ SBS block copolymer, M_(w) 170000 g/mol, 31% 38% butadiene styrene content, block 72° Shore A (KratonD-1101) copolymer Secondary Styrene/butadiene two-block copolymer, 10%binder M_(w) 230 000 g/mol (Kraton DX-1000) Plasticizers Polybutadieneoil 20% White oil 18% Components Hexanediol diacrylate 10% forPhotoinitiator 2% crosslinking Additives Dye, thermal stabilizer 2%

[0090] The components were processed as in example 1. The thickness ofthe resulting crosslinkable, laser-engravable layer was 1.14 mm.

EXAMPLE 3

[0091] The procedure was as in example 1, except that an additionalpolymer layer comprising a water-soluble polymer was also applied to therelief layer (polyvinyl alcohol, Alcotex 4-86, thickness: 3 μm). Forthis purpose, in a separate process step, the protective PET sheetmentioned at the outset was coated with a solution of Alcotex 4-86 in awater/alcohol mixture and the solvent mixture was evaporated. The coatedPET sheet was used for the extrusion process described. The thickness ofthe resulting crosslinkable, laser-engravable layer was 1.14 mm.

COMPARATIVE EXAMPLE 1

[0092] A photochemically crosslinkable laser-engravable relief-forminglayer was produced using the following starting materials AmountComponent Description [% by wt.] Elastomeric SIS block copolymer, M_(w)210 000 g/mol, 17% 48% binder styrene content, 31° Shore A (KratonD-1161) Plasticizer White oil 6% Components Hexanediol diacrylate,monoacrylate 13% for Photoinitiator 2% crosslinking Additives Dye,thermal stabilizer 4%

[0093] The components were processed as in example 1. The thickness ofthe resulting crosslinkable, laser-engravable layer was 1.14 mm.

[0094] Carrying Out the Novel Process:

[0095] The protective PET sheet was peeled off from the laser-engravableflexographic printing elements obtained in the examples and comparativeexamples. They were uniformly crosslinked by exposure to UVA light for20 minutes in a first process step. In examples 1 and 2, additionalcrosslinking of the uppermost region of the relief layer was carried outusing UVC light.

[0096] Laser Engraving of the Flexographic Printing Elements

[0097] A three-beam CO₂ laser (STK, Kufstein, type BDE 4131) was usedfor laser engraving experiments.

[0098] After the flexographic printing element had been clamped on acylinder, a test motif consisting of various, typical, positive andnegative elements was engraved into the flexographic printing element.In addition to surface areas completely removed by engraving and 100%tonal values, the motif also contained various screen areas having tonalvalues of from 1 to 98% and 40 μm wide negative lines in the axial andtransverse directions relative to the axis of rotation of the cylinder.The speed of rotation of the cylinder was 7 m/s. The power setting ofthe beams was: 1st beam 40, 2nd and 3rd beams 90.

[0099] After the laser engraving, the flexographic printing platesobtained were washed for two minutes with water with simultaneousbrushing of the surface. A nyloprint® washer (apparatus combination CW22×30, BASF Drucksysteme GmbH) was used for this purpose.

[0100] The following features are determined for assessing the qualityof the flexographic printing plates:

[0101] The engraving depth T as a measure of the sensitivity, measuredas a height difference between a part from which material has beenuniformly removed and the plate surface.

[0102] Visual assessment of the formation of deposits, melt edges andtacky droplets (deposits) and visual assessment of the possibility ofwashing away superficial deposits (washability) during subsequentwashing with water.

[0103] The results are listed in table 1.

[0104] Furthermore, FIGS. 1 and 2 each show an image of the flexographicprinting plate obtained according to comparative example 1 and accordingto example 1. TABLE 1 Engraving Example depth T Deposits Washability No.[μm] (visually) (visually) 1 410 few good 2 430 few good 3 410 few verygood C 1 300 many poor

[0105] Both the results of the measurements and the figures clearly showthat the novel process gives flexographic printing plates which havescarcely any melt edges and substantially fewer deposits than in thecomparative example. The height of the engraved relief elements issubstantially greater in the example than in the comparative example.

[0106] The flexographic printing plates obtained according to theinvention are suitable for printing with alcohol-based and water-basedinks.

EXPLANATION OF FIGURES:

[0107]FIG. 1: Flexographic printing plate acc. to comp. ex. 1

[0108]FIG. 2: Flexographic printing plate acc. to example 1

[0109] The “A” is 6 mm wide and 7 mm high in each case.

1. A process for the production of flexographic printing plates by meansof laser engraving, in which the starting material used is acrosslinkable, laser-engravable flexographic printing element which atleast comprises, arranged one on top of the other, a dimensionallystable substrate, at least one crosslinkable, laser-engravablerelief-forming layer having a thickness of at least 0.2 mm, at leastcomprising an elastomeric binder, a plasticizer and components forcrosslinking, and which process comprises at least the following steps:(a) uniform crosslinking of the relief-forming layer and (b) engravingof a print relief into the crosslinked relief layer with the aid of alaser, the height of the relief elements to be engraved with the laserbeing at least 0.03 mm, wherein the binder is a styrene/butadiene blockcopolymer having an average molecular weight M_(w) of from 100 000 to250 000 g/mol, a Shore A hardness of from 55 to 85 and a styrene contentof 20-40% by weight, based on the binder, and the amount of theplasticizer is from 20 to 40% by weight, based on the sum of allcomponents of the layer.
 2. A process as claimed in claim 1, wherein theaverage molecular weight M_(w) of the binder is from 150 000 to 250 000g/mol.
 3. A process as claimed in claim 1, wherein the styrene contentof the binder is from 25 to 35% by weight.
 4. A process as claimed inclaim 1, wherein the plasticizer is an inert plasticizer.
 5. A processas claimed in claim 4, wherein the inert plasticizer is at least oneinert plasticizer selected from the group consisting of aromatic,naphthenic and paraffinic mineral oils.
 6. A process as claimed in claim1, wherein the uniform crosslinking (a) is carried out photochemically,by means of electron beams or thermally.
 7. A process as claimed inclaim 1, wherein the relief layer additionally comprises an absorber forlaser radiation.
 8. A process as claimed in claim 1, wherein theflexographic printing element comprises an additional, water-solublelaser-engravable layer which is arranged on the laser-engravablerelief-forming layer and comprises at least one polymer soluble,swellable or dispersible in aqueous solvents and which is removed afterprocess step (b) in a further process step (c) by means of water or anaqueous cleaning agent.
 9. A process as claimed in claim 8, wherein thepolymer is at least one polymer selected from the group consisting ofpolyvinyl alcohol, polyvinyl alcohol/polyethylene glycol graftcopolymers, polyvinylpyrrolidone and cellulose derivatives.
 10. Aflexographic printing plate obtainable by a process as claimed inclaim
 1. 11. The use of a flexographic printing plate as claimed inclaim 10 for flexographic printing with water-based and/or alcohol-basedprinting inks.